Glossary of Terms


Abbreviation for asphalt cement.

Adjusting Mechanism

The first stage apron is held by gravity and can be adjusted with the nut.  The second stage apron setting can be changed by raising the aprons and adding or removing shims.


Inert material such as sand, gravel, crushed stone and slag used as the chief ingredient of HMA.

  • SAND is a fine granular material (usually less than 1/4-inch dia.) produced by natural disintegration of rock or by crushing friable sandstone rock or other suitable rocks.
  • GRAVEL is a coarse granular material (usually larger than 1/4-inch dia.) produced by natural erosion and disintegration of rock.
  • CRUSHED GRAVEL is produced by artificially crushing gravel, causing most fragments to have at least one fractured face.
  • CRUSHED STONE is produced by artificially crushing rocks, boulders or large cobblestones, causing the fragments to have all fractured faces.
  • ROCK suitable for making good aggregates is formed all over the world and used to make crushed stone, sand and gravel.
  • SLAG is the air-cooled, non metallic by-product of a blast furnace operation. It consists essentially of silicates and alumino-silicates of lime and other bases.

Anvils/Cluster Ring

Replaceable wear parts connected to the tub to provide an impact area.


Commonly used as a shortened term for asphalt cement, but may also refer to asphalt pavement.

Asphalt Cement

The binder or “glue” in HMA. Commonly abbreviated AC. At ambient temperatures asphalt cement is a black, sticky, semi-solid and a highly viscous substance. 

Asphalt cements used in HMA are made by distillation of crude oil and are available in various grades. They are more suited for HMA than asphalt made from other materials or asphalt obtained from native deposits in various parts of the world.

Use of coal tar is avoided by the HMA industry because its fumes are known to be carcinogenic, posing a health hazard for anyone working with HMA. The word bitumen is used in many parts of the world to mean asphalt cement.

Asphalt Concrete

A dense-graded HMA. Also called asphaltic concrete or bituminous concrete

Attrition Crushing

Crushing by friction, or rock on rock crushing.

Bar Grizzly

A grill or network of bars which passes material small enough to go through the spaces between the bars and withholds or separates material larger than the spaces between the bars. Grizzlies are available with various bar spacing.


The base contains the crushing chamber between the stationary jaw and the moving jaw.  Steel saddles at the top of the base support the eccentric shaft assembly and the pitman.  The lower rear section contains the adjusting mechanism and supports the tension rod assembly.  Abrasion resistant steel wear plates, bolted to the sides of the base protecting the sides of the base.

Belt Conveyor

A mechanical device that uses belts, rollers, pulleys and other hardware to move tons of virtually any material. Perhaps the most easily recognizable of all conveyor types. Belt conveyor systems provide the means of transporting materials via the shortest distance between the required loading and unloading points. When required, belt conveyors can operate continuously, without loss of time and are capable of handling tonnages of bulk materials that would be more costly and often impractical to transport by other means. This often avoids confusion, delays, and safety hazards of rail and motor traffic in plants and other congested areas.

Belt Feeder

A belt-conveyor that feeds aggregate or RAP from its bin onto another conveyor. It often has a precision variable-speed drive to ensure accurate mix formulas. A variable-speed drive also permits changing the production rate without changing the mix formula.

A tachometer on the tail shaft gives a reliable indication of belt speed to ensure accurate blending, even if the belt slips on the head shaft

Belt Scale

A scale usually incorporated in the incline conveyor of continuous mix HMA facilities. It has a load cell that accurately indicates the weight imposed by a section of the belt running over a weigh idler. This information combined with the belt speed denotes how much material is entering the dryer drum.  The scale incorporates a test weight for initial calibration. Material calibration is done by running sample material across the scale. The sample is diverted to a container and weighed on an independent scale. Scale indications are compared. If necessary, the belt scale is calibrated to agree with the test scale.


Improvement of the chemical or physical properties of a material or intermediate product by the removal of undesirable components or impurities or the addition of desirable components.


Used in many parts of the world to mean asphalt cement. (See asphalt cement.)

Blow Bars

Are attached to the rotor and impact the material, propelling it outward toward the aprons to be further reduced in size.  These bars are reversible to increase service life.

Bowl Float

When the forces in the crushing chamber are greater than the tramp iron relief cylinder hold down force the upper assembly will lift up off the V-seat. Micro-float is very destructive bowl float with minimal displacement.


Abbreviation for cubic feet per minute.

Choke Feed

Common term used to describe rock covering the cone by 6” and 360 degrees around the cone head. Contributes to rock on rock crushing (attrition crushing), even liner wear and maximum liner life.  Prevents bowl float.

Closed Circuit

Crushed rocks return to the screen. Any material not passing returns to the crusher (recirculating load).

Closed Side Setting

Crusher setting when the discharge opening of the jaw crusher is at closest position referred to as CSS.


The wire mesh material used in a scalping screen. (See scalping screen.)

Cold Planer

Also called a milling machine. A mobile machine that removes old paving from a roadway by planing or milling. It has cutting teeth affixed to a rotating drum. It also has a belt conveyor that loads the removed material into a truck that moves along the road ahead of the machine.


A hard, strong manmade construction material consisting of sand, conglomerate gravel, pebbles, broken stone, or slag in a mortar or cement matrix. It is commonly used for building roads, bridges and architectural structures.

The word concrete is sometimes used with other words to designate another type of manmade construction material, for example asphalt concrete.

Note that concrete does not always refer to manmade construction materials. It can refer to substances formed in nature by the coalescence of separate particles or parts into one solid mass. An example is concreted earthy or mineral matter, such as rock.


To make impure or unclean by contact or mixture.


Jargon for shim. (See shim.) The term cribbing is sometimes used to mean a wall of timbers that confine earth and serve as a retaining wall.

Crushing Chamber

Space between the stationary and the moving jaw.


A platform that contains a screen used to separate materials. Cold feed systems normally use one or two screen decks. RAP systems normally use only one. Batch towers normally use three-and-one-half. Additional decks can be added if needed.


To lower in quality.


To remove water by pumping, drainage or evaporation or a dewatering screw.

Eccentric Shaft

The eccentric shaft translates circular motion into the crushing action of the moving jaw.  The shaft revolves in two roller bearings mounted at the top of the frame.  The center portion of the shaft is eccentric and carries the pitman on two inner roller bearings.

Expansion Chamber

Space between the rotor and aprons.

Feed Disc

Provides even material distribution to the shoes.

Feed Tube

Provides the transition onto the Feed disc from the feed box.


Relatively short conveyor that “feeds” and controls delivery to conveyors, processing units and other equipment.

Feeder Belt

(See Belt Feeder)

First Stage Apron

This apron deflects uncrushed particles back into the rotor (a.k.a. primary curtain).


An open conduit of wood, concrete or metal.


The flywheels store energy to provide smoother operation when surges of material enter the crusher.  One of the two flywheels is grooved for V-Belts, to drive the crusher.


Main structure for support.


The multiples of the acceleration of gravity. Horizontal screens run at a maximum of 8.14 g. Clears material from media. High G prevents blinding.


Top opening dimension of the jaw crusher from the stationary jaw to the pitman.


Abbreviation for gallons per minute.


A sequence of gradual, successive stages of rock sizes.

Gravity Take-Up

Can be located anywhere along the length of the conveyor, generally used on conveyors greater than 150’ long with higher HP’s.


(See Bar Grizzly)

Grizzly Section

Designed to remove unwanted material from entering the crusher.


Abbreviation for hot mix asphalt.


A sloped or funnel shaped container in which materials are collected to facilitate transferring them to another location.

Hot Mix

Hot mix asphalt.

Hot Mix Asphalt (HMA)

Road paving material produced by mixing hot dry aggregate and liquid asphalt cement. Basic types of HMA are dense-graded and open-graded. There are sub-types within the basic types.

The mixture is made with the ingredients heated to about 300 degrees F, which is the temperature normally used to dry the aggregate. The liquid asphalt cement will adhere to the aggregate only if it is dry. The mix may also include dust or fines from the aggregate, ground rubber, and additives fillers, such as lime. The mixture is maintained close to 300 degrees until it is applied to the road.

HMA is called by numerous other names, such as asphalt concrete, asphaltic concrete, asphalt cement concrete, asphalt mix(ture), asphalt paving mix(ture), bituminous concrete, bituminous mix(ture), bituminous paving mix(ture), etc.

HSI Crusher

Horizontal Shaft Impact crusher that breaks material by impact.

Hydraulic Main Access

This system allows the crusher housing to be opened to provide access to the rotor and internal components.


Hardware that supports the conveyor belt and the material being moved.  Generally made with rolls of steel or cast iron and supporting roller and ball bearings.

Impact Crusher

A machine commonly used to crush material in stone quarries or metallic mines. They are also used in recycling asphalt or concrete.

One type of crusher is known as a horizontal shaft impactor (HSI). It has long bars, known as blow bars or hammers, fixed on a rotor so they cannot rebound. The bars strike the material breaking it and throwing it against hinged aprons which cause further breakage.


(See Impact Crusher)

Impeller Shoes

Mounted on the table provides acceleration of the material.

Jaw Crusher

A crusher that breaks material by squeezing it between two Jaw plates, one moving and one stationary.

Jaw Dies

The jaw dies provide a hardened steel surface to crush the rocks against.  The stationary jaw fits on the inside front face of the base and the moving jaw is bolted to the pitman.  Jaws are reversible top to bottom for extended jaw life.

Liquid AC

(See Liquid Asphalt Cement)

Liquid Asphalt Cement

Asphalt cement that is heated to a liquid state to make it suitable for mixing with aggregate to make HMA. At ambient temperatures, asphalt cement is a semisolid. (See asphalt cement.)

Loading Zone

Area that will receive the incoming material.

Lower Rollers

Support the pans on the underside of the apron feeder.


An early paving material consisting of compacted broken stone. Now, it is usually bound with tar or asphalt. The term is often used synonymously with hot mix asphalt, but is a somewhat antiquated term. [After John Loudon McAdam (1756-1836), Scottish civil engineer.]

Manufactured Sand

Not natural occurring sand, -3/8” material made by crushing +3/8” material.


The number of openings per lineal inch in wire screen.  Example - #4 mesh = four (4) approx. 3/16” openings per lineal inch.


A unit of length; one thousandth of a millimeter.

Milling Machine

Also called a cold planer. (See Cold Planer)


A natural, homogeneous, inorganic, solid substance used as the main ingredient of HMA. A wide variety of minerals can be used in HMA, but the main ones used are sand and stone. The materials available vary widely according to geographical location. The following materials are used in HMA in various parts of the world:

  • soft limestone
  • limestone
  • dolomite
  • sandstone
  • granite
  • quartzite
  • trap rock
  • gravel
  • granite gravel
  • slag

Open Circuit

Crushed rocks do not ever return to the crusher.

Open Side Setting

Crusher setting when the discharge opening is at its furthest open position.


Soil mantle, waste, or similar matter found directly above a deposit of rock or sand and gravel.

Pan Deck

A Horizontal plate on the feeder. Provides the base for stratification.


Formed plate bolted and interlock on to the feeder chain.


Mechanical connection between the accelerator and the power unit.


The pitman carries the moving jaw and moves in an oval path from the rotation of the eccentric shaft.


Natural gravel deposits; may contain some sand, clay or silt.


All conveyors have a minimum of a head pulley and a tail pulley, belting wraps around the pulley.


When used as a verb, recycle means to reuse. When used as a noun, recycle is jargon for RAP.

Recycle Material

Commonly used to mean RAP, but could also mean RAM.


Reuse of reclaimed asphalt pavement in construction of new roadways and other structures.

Reduction Ratio

Ratio relating to the top size of the feed to the CSS of the crusher. Divide the top size by the CSS to obtain the reduction ratio (e.g. 6” top size/1” CSS = 6:1 ratio).

Rock Shelf

Enable material to build up in front of the shoes to provide rock on rock crushing.


The rotor is the heart of an impactor and is responsible for the majority of the crushing action.  Used in semi and fully autogenous crushing. Replaces the table and shoes.


Standard classification of soil or granular material passing the 3/8” (9.52 mm) sieve and almost entirely passing the No. 4 (4.76 mm) sieve and predominantly retained on the No. 200 (74 micron) sieve.

Scalping Screen

A wire mesh screen, which passes material small enough to go through the spaces between the wires and withholds or separates material larger than the spaces between the wires. Screens with various mesh sizes are used in making HMA.

Screen Media

The surface attached to the screen decks to size the material on the screen deck.

Screen Mesh Sizes

(See Mesh Numbers)

Screw Take-Up

Located at the tail pulley, generally used on conveyors less than 150’ long with low HP.

Second Stage Apron

The second stage apron is responsible for the rest of the crushing not done by the rotor.


An unwanted separation of larger sized aggregate from smaller sized aggregate after the aggregates have been mixed with each other.

Segregation usually occurs as a result of moving mixed material from one place to another. When mixed material flows into a pile, the larger pieces tend to roll off the top of the pile to one side while the smaller pieces remain near its center. This, of course, segregates the material. Special techniques are used to avoid segregation.


To insert timbers or other materials under a leg or support pad of equipment in a HMA facility to make it level or to adapt it to an uneven surface.


Roofing material containing asphalt materials that can be recycled for use in HMA.


Test screens with square openings.


Minus #200 mesh material.

Specific Gravity

The ratio of the mass of a unit volume of a material at a stated temperature to the mass of the same volume of a gas-free distilled water at the same temperature.


A pile of material, usually aggregate or RAP, used as an ingredient of HMA. Stockpiles are usually created and maintained by dump trucks unloading material into piles on the ground. In most facilities, front-end loaders scoop up material from the stockpiles and empty it into either cold feed or RAP bins.


Separation of material by lump size.


Provides the mounting of the shoes.


Means to keep proper belt tension.

Tension Rods

The tension rod assembly is a threaded rod extending through a spring.  This assembly pulls the pitman back against the toggle plate, holding it in place.

Timing Angle

The angle from vertical of the magnitude of the stroke.


The Tramp Iron Relief system protects the rock crusher from damage due to tramp iron or overload.

Toggle Plate

The toggle plate positions the lower end of the pitman.  Although the toggle plate will sometimes fall in the event of an extreme overload, this does not always occur and cannot be counted on as safety device.


Abbreviation for tons per hour. Unless noted otherwise, a ton in Astec publications refers to a short ton (2,000 pounds.)

Tramp Iron

Any non-crushable in the chamber (e.g. loader bucket tooth).


Outer structure of the crushing chamber.

Upper Rollers

Support the chains connected to the pans.

Variable Speed Drive

A variable speed drive system used for belt feeders. (See belt feeder.)


Vibrating Grizzly Feeder.

Virgin Aggregate

Aggregate not previously used in paving products.

Virgin Asphalt

Asphalt cement not previously used in paving products.


The measure of the ability of a liquid or solid to resist flow.  A liquid with high viscosity will resist flow more readily than a liquid with low viscosity.


Spaces between grains of sand, gravel or soil that are occupied by water or air or both.

VSI (Vertical Shaft Impactor)

Material is fed to the center of the crusher directed through an accelerator. As the material passes through the accelerator it is given a high amount of inertia “stored energy.”


Material not suitable for inclusion in a particular HMA mix.


A structure for diverting or measuring the flow of water.

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